Mechanisms of Heat Transfer Enhancement in a Turbulent Boundary Layer Downstream of a Cylinder-Wall Junction

The wall heat transfer enhancement behind a circular cylinder-wall junction was investigated experimentally for Reynolds numbers ranging from ReD = 21,000 to 54,000 for locations up to 12 diameters downstream of the cylinder leading edge. Surface heat transfer was studied using a fully-heated surface downstream of the cylinder to provide traditional heat transfer coefficients, and thin-film surface sensors flush-mounted on an unheated surface to provide adiabatic heat transfer coefficients. Flow field transport measurements were obtained with a triple-wire Reynolds heat flux probe. The enhancement could be attributed to two effects: (1) a local fluid dynamic effect attributed to increased eddy diffusivity and subsequent increased turbulent transport and (2) an upstream heating effect, caused by reduced wall temperatures in the region directly behind the obstacle, and their effect on the subsequent redevelopment of the boundary layer downstream. The adiabatic heat transfer coefficients obtained from the surface sensors provided misleading results, because of the breakdown in the analogy between heat and momentum transport.